Diesel engines, because of the way they operate, emit soot particles or very fine droplets of condensate or a conglomerate of the two (particulates) as well a typical harmful gasoline engine exhausts (i.e., HC and CO). These “particulates” (herein Diesel soot), are rich in condensed, polynuclear hydrocarbons, some of which may be carcinogenic. As the awareness of the danger Diesel soot presents to health collides with the need for greater fuel efficiency that Diesel engines provide, regulations have been enacted curbing the amount of Diesel soot permitted to be emitted. To meet these challenges, soot filters have been used. When using such a filter, the filter must be periodically regenerated by burning off the soot. This burning of soot results in stresses from axial and radial temperature differences that can cause cracking of the filter.
To overcome stresses, ceramic honeycombs such as heat exchangers and filters have reduced the stresses and potential for cracking the honeycombs by assembling smaller honeycombs into larger honeycombs. Cement layers between the honeycombs have been used, for example, to increase the thermal conductivity to reduce the ultimate temperature reached in the assembled honeycomb such as described by EP 1508355. The dimensions of the cement layers between the segments are important to the performance of the larger assembled honeycombs. If they are too thin, there may be inadequate adhesion and mechanical failure during use of the honeycombs. If they are too thick, there may be excessive thermal gradients during heating and cooling of the honeycomb which can also result in mechanical failure of the honeycombs. Further if the seams are too thick the honeycomb may exhibit high back pressures, which can negatively impact the performance of the honeycombs and the systems utilizing the honeycombs.
It is known to use spacers to control the thickness of the adhesive layer in the assembled ceramic parts. Such spacers may remain in the adhesive layers once the assembled ceramic parts are completed or they need to be removed by burning them out in a burn-out process. In other processes special apparatus are required for assembly which contain the spacers in the assembly apparatus. See for instance EP 1,435,348; EP 1,612,197; US 2006/0254713; and US 2008/0014404 all incorporated herein by reference in their entirety. Such processes require special assembly steps, burn out steps or leave spacers and/or the residue of the spacers in the seams which can reduce the effectiveness of the resulting ceramic filters.
Thus, what is needed are methods of assembling arrays of ceramic filters which is elegant, avoids the use of specialized spacer systems, avoids leaving spacers or the residue of spacers in the arrays and which does not detract from the ultimate performance of the resulting arrays of ceramic parts.